The reverse osmosis phenomenon is widely used in under-the-sink apparatus for making "purified" drinking water available at a faucet mounted on the sink top. The process is practiced with the aid of a membrane whose usefulness is limited and which must be replaced from time to time. The need for replacement is a function of the history of impurity removal rather than time or taste. However, in the absence of a means for measuring that history or of measuring taste it is common for manufacturers of reverse osmosis machines to recommend changing membranes periodically. In practice the time interval between change is so long that the time for change is easily forgotten or the need for service has itself been forgotten. The result in such cases is that membranes are changed more often than needed or not often enough.
In the case of drinking water "purification", the permissible magnitude of impurity content is established arbitrarily. In the case of water supplied by municipal or commercial organization, the value ordinarily is set by the purification system manufacturer or by the installer at some value less than the water supplier's maximum content standard.
Total impurity content is difficult to measure but finding the content of electrically conductive material in relative terms is not. It is common to measure water purity as a function of the electrical conductance between a pair of electrodes spaced a fixed distance apart while immersed in the water to be evaluated. While the measurement is easy to make with a simple conductivity meter, power is required to make and display the measurement. If the user must remember to apply that power and to make the measurement periodically, little has been gained and the opportunity for failure to detect when a membrane change is to be made is as great as in the past. An alternative is to apply power continually and to provide a continuous indication of the water's conductivity. To do that raises other problems. Continuous indication may require so much power that using batteries as the power source may be impractical. On the other hand, to employ alternating "house" current requires rectification and current control and safety measures which are generally cost prohibitive. One solution has been to employ battery power and an intermittent indication and display. It is the purpose of this invention to provide better solutions to the problem.